Trucks and other vehicles that are intended to carry heavy loads necessarily have relatively rigid springs. While these springs perform satisfactorily at the upper end of the load carrying capacity range of the vehicle, they can be ineffective at the lower end of the range. When the vehicle is empty, vibrations and abrupt wheel movements caused by the unevenness of the road surface can be transmitted to the vehicle frame, almost as if the vehicle were operated without springs.
It is known that the above problem can be overcome by the employment of an auxiliary spring arrangement such as that known as the Empty Ride.RTM. system of the Cambria Spring Co. The hangers that secure the mainspring to the frame are modified in such a way that the mainspring can move, at least at one end, within a limited range of vertical travel. The mainspring is then biased toward the lower limit of its vertical travel by a smaller and lighter auxiliary spring positioned between the mainspring and the frame. Thus, in essence, the lightly loaded or empty vehicle rides on the auxiliary spring which has a spring rate appropriate for these conditions. When the vehicle is heavily loaded, however, the auxiliary spring is deflected sufficiently to move the mainspring to the upper limit of its vertical travel, and the vehicle is then dependent upon deflection of the mainspring to isolate the frame from road surface variations. Suspension systems of this type are described in previously issued U.S. Pat. Nos. 3,194,580 to Hamlet and 4,175,772 to Lampert.
The present state of the art in the development of auxiliary springs systems of the type described to above is generally satisfactory for systems of the type referred to herein as "wide ratio systems". In these systems the weight of the loaded axle to which the auxiliary spring is many times that of the empty vehicle. It is therefore possible, using previously known techniques, to fit an auxiliary spring having the desired spring rate between the mainspring and the frame. A persistent problem, however, to which a satisfactory solution has been sought for some time exists with respect to "close ratio systems" in which the weight of the unloaded vehicle, with respect to the axle in question, is relatively close to that of the unloaded vehicle. Since the mainspring and the auxiliary spring must be comparatively similar in size, it is often impossible to fit the auxiliary spring into the space available.
An important application of close ratio systems relates to the front axle of a tractor of the type commonly used to pull a semi-trailer. Only a small portion of the weight of the semi-trailer is transmitted to the front axle. Typically, the weight borne by the front axle of a loaded rig would be about double the weight borne by the front axle of the unloaded rig.
While the problem presented in this close ratio situation might at first appear to be less severe than the problems encountered with respect to the trailer in which the load fluctuations are much greater, it should be remembered that the comfort of the driver and the susceptibility of the driver to fatigue on long hauls must be considered. In fact, driver discomfort and fatigue in unloaded rigs is a major problem in the trucking industry. Moreover, the effectiveness of the suspension system with respect to the front axle relates directly to the performance of the steering system of the vehicle.
A principal objective of the present invention is to provide an auxiliary suspension system that can be readily employed in close ratio environments to improve the ride characteristics of a vehicle. Another objective is to provide an auxiliary spring suspension system which is adaptable to use in those situations in which only limited space is available for the installation of the auxiliary spring.